Inks are often of one of two types, depending on the type of colorant used. Dye-based inks often comprise a dye dissolved in a liquid vehicle. Pigment inks comprise a pigment dispersed in a particulate form in a liquid vehicle. Pigment inks tend to have better ozone-fastness and light-fastness than dye-based inks. However, because the pigment is in the form of fine particles there is a tendency for the particles to agglomerate or flocculate whilst the ink is being stored and/or whilst the ink is being used (e.g. printed). Such agglomeration or flocculation before the ink has been printed onto a substrate is highly undesirable, particularly in ink jet printing inks where the printer nozzles are very small and are susceptible to blockage by any oversized particulate matter. Thus, in the ink jet field a great deal of effort has been spent attempting to provide sub micron sized pigment dispersions and increase the colloidal stability of these pigment dispersions.
It is also desirable to provide pigment inks which offer high optical density (OD), especially when printed onto plain paper.
Pigment dispersions are often colloidally stabilised by means of a dispersant.
It is desirable that a dispersant is effective in the dispersion or comminution process. Quicker comminution (e.g. milling) to submicron particles sizes saves substantial energy and it may also result in less pigment particles having a particles size markedly smaller than the target size. Particles much below the target size are often referred to as fines.
Pigment based inks when printed onto a substrate often produce final images that are not firmly secured to the substrate surface. Prints from pigment based inks often exhibit poor rub fastness, water-fastness or highlighter smear fastness.
In our own studies C.I. Pigment Yellows and especially C.I. Pigment Yellow 155 and C.I. Pigment Yellow 74 have proved to be particularly difficult to disperse to a suitably small particle size for preparing ink jet printing inks. In our studies the milling times required for the preparation of C.I. Pigment Yellow 74 dispersions with an average particle size of around 100 nm can often be twice as long as those for cyan, magenta and black pigments at the same average particle size. This represents a substantial cost in providing the required milling energy.
It is known that dispersions of C.I. Pigment Yellow 74 pigments tend to have relatively poor colloidal stability resulting in flocculation, particle size growth and sedimentation whilst being stored and/or whilst in the printer. Such is recognised by for example PCT patent publication WO 2010/068776.
In our studies we have seen that the colloidal instability of the dispersions are more pronounced at elevated storage temperatures, over longer storage times and in liquid vehicles which contain smaller proportions of water relative to water-miscible organic liquids.
The above problems limit the performance and adoption of C.I. Pigment Yellows in ink jet printing inks.
Commercially, there still remains a need for dispersants which can be used to prepare pigment inks (especially inks containing the pigments C.I. Pigment Yellow 74 and 155) which solve, at least in part, one or more of the abovementioned problems.